This paper reports on a range of SLM applications we have studied in femtosecond processing of transparent glasses and thin films. Laser phase-fronts were tailored by the SLM to compensate for spherical surface aberration, and to further address the nonlinear interactions that interplay between Kerr-lens self-focusing and plasma defocusing effects over shallow and deep focusing inside the glass. Limits of strong and weak focusing were examined around the respective formation of low-loss optical waveguides and long uniform filament tracks. Further, we have employed the SLM for beam patterning inside thin film, exploring the limits of phase noise, resolution and fringe contrast during interferometric intra-film structuring.

Femtosecond laser pulses of 200 fs pulse duration and 515 nm wavelength were shaped by a phase-only LCOS-SLM (Hamamatsu X10468-04). By imposing radial phase profiles, axicon, grating and beam splitting gratings, volume shape control of filament diameter, length, and uniformity as well as simultaneous formation of multiple filaments has been demonstrated. Similarly, competing effects of spherical surface aberration, self-focusing, and plasma de-focusing were studied and delineated to enable formation of low-loss optical waveguides over shallow and deep focusing conditions.

Lastly, SLM beam shaping has been successfully extended to interferometric processing inside thin transparent film, enabling the arbitrary formation of uniform or non-uniform, symmetric or asymmetric patterns of flexible shape on nano-scale dimensions without phase-noise degradation by the SLM patterning. We present quantized structuring of thin films by a single laser pulse, demonstrating λ/2nfilm layer ejection control, blister formation, nano-cavities, and film colouring. Closed intra-film nanochannels with high aspect ratio (20:1) have been formed inside 3.5 um thick silica, opening new prospects for sub-cellular studies and lab-in-film concepts that integrate on CMOS silicon technologies.

Interferometric femtosecond laser processing of thin dielectric films has recently opened the novel approach for quantized nanostructuring from inside the film, driven by the rapid formation of periodic thin nanoscale plasma disks of 20 to 45 nm width, separated on half-wavelength, λ/2nfilm, spacing (refractive index, nfilm). The nano-disk explosions enable intra-film cleaving of subwavelength cavities at single or multiple periodic depths, enabling the formation of intra-film blisters with nanocavities and the digital ejection at fractional film depths with quantized-depth thickness defined by the laser wavelength.

For this paper, the physical mechanisms and ablation dynamics underlying the intra-film cleavage of SiOx thin films were investigated by laser pump-probe microscopy with high temporal dynamic range recorded in a wide time-frame between 100 fs and 10 μs. The long time scales revealed a new observation method as Newton's Rings (observed <~50 ns) gave way to holographic recording (>~50 ns) of the laser-ablated film fragments. For the first time to our knowledge, the holographic tracking reveals the clustering of large mechanically ejected nano-film planes into distinct speed groups according to the multiple of λ/2nfilm in the film. The observation verifies a new ‘quantized’ form of photo-mechanical laser “lift-off”.

Multifunctional lab in fiber technology seeks to translate the accomplishments of optofluidic, lab on chip devices into silica fibers. a robust, flexible, and ubiquitous optical communication platform that can underpin the ‘Internet of Things’ with distributed sensors, or enable lab on chip functions deep inside our bodies. Femtosecond lasers have driven significant advances in three-dimensional processing, enabling optical circuits, microfluidics, and micro-mechanical structures to be formed around the core of the fiber. However, such processing typically requires the stripping and recoating of the polymer buffer or jacket, increasing processing time and mechanically weakening the device. This paper reports on a comprehensive assessment of laser damage in urethane-acrylate-coated fiber. The results show a sufficient processing window is available for femtosecond laser processing of the fiber without damaging the polymer jacket. The fiber core, cladding, and buffer could be simultaneously processed without removal of the buffer jacket. Three-dimensional lab in fiber devices were successfully fabricated by distortion-free immersionlens focusing, presenting fiber-cladding optical circuits and progress towards chemically-etched channels, microfluidic cavities, and MEMS structure inside buffer-coated fiber.

The nonlinear interactions of femtosecond lasers are driving multiple new application directions for nanopatterning and structuring of thin transparent dielectric films that serve in range of technological fields. Fresnel reflections generated by film interfaces were recently shown to confine strong nonlinear interactions at the Fabry-Perot fringe maxima to generate thin nanoscale plasma disks of 20 to 40 nm thickness stacked on half wavelength spacing, λ/2nfilm, inside a film (refractive index, nfilm). The following phase-explosion and ablation dynamics have resulted in a novel means for intrafilm processing that includes ‘quantized’ half-wavelength machining steps and formation of blisters with embedded nanocavities. This paper presents an extension in the control of interferometric laser processing around our past study of Si3N4 and SiOx thin films at 515 nm, 800 nm, and 1044 nm laser wavelengths. The role of laser polarization and incident angle is explored on fringe visibility and improving interferometric processing inside the film to dominate over interface and / or surface ablation. SiOx thin films of 1 μm thickness on silicon substrates were irradiated with a 515 nm wavelength, 280 fs duration laser pulses at 0° to 65° incident angles. A significant transition in ablation region from complete film removal to structured quantized ejection is reported for p- and s-polarised light that is promising to improve control and expand the versatility of the technique to a wider range of applications and materials. The research is aimed at creating novel bio-engineered surfaces for cell culture, bacterial studies and regenerative medicine, and nanofluidic structures that underpin lab-in-a-film. Similarly, the formation of intrafilm blisters and nanocavities offers new opportunities in structuring existing thin film devices, such as CMOS microelectronics, LED, lab-on-chips, and MEMS.

Laser welding of optical glasses remains a challenging area today because of the weak optical absorption typically available with most commercial lasers and the brittle nature of glass. In this paper, we demonstrate for the first time to our best knowledge, the laser welding of telecommunication optical fiber onto a fused silica substrate. The 157-nm F2 laser was selected for the wide processing window that drives strong absorption at high fluence exposure > 1 J/cm2 without inducing microcrack formation. The method of second surface ablation was applied to the contact point between the glass plate and glass fiber to locally heat, melt, and reflow the glass and thereby weld together the two similar glasses. Mechanical pressure was applied while the laser beam was scanned along the sample contact to produce a line of overlapping welds of 25-um spot size each. Fused silica samples of up to several hundreds of microns thick could be welded owing to a large 157-nm penetration depth of 1/a ≈ 1 mm. A narrow 3.31 to 3.66 J/cm2 fluence window was found for laser welding through 160-um thick fused silica substrates. The F2-laser welding window is constrained by the need for sufficient transmitted fluence to melt the interface without too much fluence that will damaged the interface structure at the onset of ablation or induce front surface ablation.

High-repetition rate (>200 kHz) ultrafast lasers drive novel heat accumulation processes attractive for rapid writing of low loss optical waveguides in transparent glasses. Heat accumulation is significant at ~1 MHz when thermal diffusion is insufficient to remove the absorbed laser energy from the focal volume (<10-μm diameter) during the interval between pulses. A novel femtosecond fiber laser system (IMRA, FCPA μJewel) providing variable repetition rate between 100 kHz and 5 MHz was applied to waveguide writing in fused silica and various borosilicate glasses in order to investigate the relationship of such heat accumulation effects. Waveguides were formed with <400-fs pulses at 1045-nm at pulse energies of 2.5 μJ at 100 kHz to >150 nJ at 5 MHz. Wide variations in waveguide properties were encountered, particularly when processing 1737F and AF45 borosilicate, at repetition rates greater than 200 kHz. Waveguide characterization revealed unique material-dependent thresholds for cumulative and single pulse phenomenon. Of these materials, fused silica is unusual in resisting waveguide formation at the fundamental wavelength of 1045 nm, but amenable to waveguide writing at the second harmonic of 522 nm. Laser processing windows are presented for several silica-based glasses for creating symmetric waveguides with low insertion loss when coupled to standard optical fibers. The effects of material and laser parameters on thermal accumulation and waveguide characteristics are discussed.

Direct waveguide writing with femtosecond lasers can be divided into two general categories based upon the type of lasers used: amplified systems that emit high pulse energy (>2 μJ) at low repetition rates (<250 kHz), and oscillators that produce low energy (<200 nJ) at high repetition rates (>1 MHz). In this presentation, we report on waveguide writing with a novel commercial femtosecond fiber laser system (IMRA, FCPA μJewel) that bridges the gap between these two regimes, providing sub-400 fs pulses with pulse energies of >2.5 μJ at 100 kHz and >150 nJ at 5 MHz. The laser repetition rate can be varied from 100 kHz to 5 MHz in 1 kHz increments through a computer controlled user interface. The ability to quickly and easily vary the repetition rate of this laser was critical in identifying and optimizing laser processing windows for different target glasses. An overview of laser processing windows and waveguide characteristics are presented for borosilicate and fused silica glasses exposed to fundamental (1045 nm) and second harmonic (522 nm) laser light.

Lasers microprocessing is attractive for the custom fabrication of novel lab-on-a-chip designs. However, processing of glass biochips is challenging for most lasers because of the weak light interactions inherent in such transparent substrates. The F2-laser generates a high 7.9-eV photon energy that drives strong absorption in glasses, while the short 157-nm wavelength offers high-resolution patterning on the 100-nm scale. With these benefits, F2-laser ablation is well suited to the fabrication of high aspect ratio microfluidic channels and other biochip functions. F2-laser radiation also produces a strong photosensitivity response in fused silica and other glasses that enable the fabrication of buried optical waveguides, Bragg grating filters and other refractive index structures inside the glass. In this paper, we combine laser micromachining and refractive index profiling to enable single-step integration of photonic functions with microfluidic functions on a single chip. Optical waveguides were written to intercept microfluidic channels for optical sensing of cells and other bio-materials. An integrated biophotonic sensor is demonstrated for polystyrene spheres. The sensor is optically characterized for insertion loss, propagation loss, and particle sensitivity. The demonstration and analysis of this simple device offers insight into the capabilities and potential applications for laser fabricated glass lab-on-a-chip devices. Moreover, the groundwork is laid for rapid laser prototyping of custom-designed microfluidic biochips interlaced with integrated-optical circuits to define a new generation of highly functional bio-sensor and lab-on-a-chip devices.

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Journal of Applied Remote SensingJournal of Astronomical Telescopes Instruments and SystemsJournal of Biomedical OpticsJournal of Electronic ImagingJournal of Medical ImagingJournal of Micro/Nanolithography, MEMS, and MOEMSJournal of NanophotonicsJournal of Photonics for EnergyNeurophotonicsOptical EngineeringSPIE Reviews